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RE: [[OT] Rear Wing Aerodynamics - trivial...NOT!!



Actually, the NB and Audi TT shape, if you look at it, is an upside-down
aircraft wing...almost a perfect flat-bottom teardrop shape. This actually
creates LIFT.

Drag has nothing to do with the wing on the back of the NB. That wing pops
up at over 80mph because the shape of the car will actually pick the rear
wheels off the ground at high speed! The sunroof also closes automatically
at that speed for the same reason (the empty space creates an additional
low-pressure area on the roof). The TT suffers less from this effect because
it's cross-section is not as thick, so it produces somewhat less lift.
However, TT's have been in some rather nasty accidents at high speeds in
Europe. Audi has since made the fixed rear spoiler an "optional recall" on
the cars. Some people have called for a mandatory recall because lives have
been lost, but Audi thinks the spoiler ruins the lines of their flagship
design and won't do it.

Most aerodynamic aids on cars, if you look at them, are NOT designed to
reduce drag. Drag is heavily influenced by the overall shape of the car,
especially the frontal area. The smaller the frontal area, the less drag.
Thus, ADDING parts almost never reduces drag (though it can in some cases,
as Cathy pointed out). Think about it: how can you stick something in the
wind, where there was nothing there before, and have it actually reduce
drag??

Most aerodynamic aids are designed to reduce lift, which actually increases
drag in most cases.  Front spoilers are a perfect example. Front spoilers
add significantly to the frontal area of a car, creating a lot of drag.
However, they keep as much air as possible from going below the car,
reducing the effectiveness of the flat-bottom teardrop shape mentioned
above. If the rear of the car is more open than the front, it will actually
create a low-pressure zone under the car resulting in a "suction". This is
why low-hanging rear valences are never anything more than cosmetic, and why
F1 cars have those big diffusers sticking out the rear. Those diffusers
start as small channels in the nose of the car, and grow to be large
cavities exiting the rear of the car, creating this low-pressure zone.
Another interesting tidbit is that F1 cars aerodynamically DIRTY. It takes
much more power to push an F1 car at 150mph than it would take to push our
Sciroccos at that speed. However, every last little bit of drag created is
for a purpose: to be converted to downforce!

Neal

> -----Original Message-----
> From: owner-scirocco-l@scirocco.org
> [mailto:owner-scirocco-l@scirocco.org]On Behalf Of Cathy Boyko
> Sent: Friday, September 21, 2001 8:09 PM
> To: tberk@mindspring.com; scirocco-l@scirocco.org
> Subject: Re: [[OT] Rear Wing Aerodynamics - trivial
>
>
>
> Super aerodynamic, I have thought about it over the years and conclude
> it is not enough to split the wind (Hey, I didn't mean cut the cheese!)
> but it's also important to close it up behind you as you pass by.
>
> Hence the needle shape at both ends of really fast things, like the SR71
> Blackbird airplane.
>
> So, Scirocco content at last, part of the benefit the rear wing, with
> it's sharp edge is to get the flow to combine back again, to avoid too
> much turbulence back behind the car as it swirls and creates a vacuum.
>
>
> >>>>And on an opposite note, I saw a post on the Beetle forum
> stating that a
> second cosmetic wing on the 1.8T (which already has a functional spoiler
> integrated into the rear window) was NOT recommended because the
> additional
> turbulence could flip the car (that's just nuts..) But I have no
> doubt that
> those cars have smoother air behind them with just the factory
> spoiler. There
> was also a good article in an old Car and driver suggesting that many
> (cosmetic)factory wings and spoilers actually slowed cars down due to the
> increased drag. (and had no additional traction benefit with the
> power coming
> from the front wheels) Hopefully our beloved black shelves are
> not among them!
> Cathy
>
> --
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